1. Field of the Invention
The present invention relates to color processing for converting a first gamut into a second gamut.
2. Description of the Related Art
In recent years, digital imaging apparatus such as digital cameras, image scanners, and the like have prevailed, and digital images can be readily acquired. On the other hand, the full-color hard copy technique is evolving at a rapid pace. Especially, printing using an ink-jet system can assure print image quality equivalent to silver halide photos, and is popularly used. Networks such as the Internet and the like have prevailed, and many users are in an environment in which they can connect various devices to the network. In such environment with diversified input and output devices, there are many opportunities to input and output color image data between devices having different gamuts. For example, a hard copy of a color image on a monitor with a broad gamut is formed by a printer having a different gamut.
As a technique for attaining identical color reproduction between devices having different gamuts, a color management system (to be referred to as “CMS” hereinafter) is prevalent. FIG. 1 is a view showing an overview of the arrangement of this CMS, and shows the CMS which uses a device-independent color space.
FIG. 1 shows an example in which input devices (a camera, scanner, and the like) and output devices (a printer, monitor, and the like) are connected. In this case, conversion from a color signal of the input system into that of the output system is implemented via profiles of the devices and a profile connection space (PCS). Note that the PCS is a device-independent color space, and for example, CIEXYZ, CIELab, and the like are available. Each profile is provided as a lookup table (LUT) as a conversion table which describes conversion formulas that connect respective device colors and the PCS or the relationship between device colors and the PCS.
FIG. 2 is a block diagram showing the basic arrangement of the CMS.
Referring to FIG. 2, an image processing apparatus 201 is a computer apparatus which executes color processing and the like associated with the CMS. An image input device 202 is a device such as a camera, scanner, or the like which inputs an image to the image processing apparatus 201. An image display device 203 is a device such as a monitor which displays an image. An image output device 204 is a device such as a printer which prints out an image supplied from the image processing apparatus 201.
In the image processing apparatus 201, an image input unit 205 inputs an image from the image input device 202. An image display unit 206 generates a signal required to display an image on the image display device 203. A color matching processor 207 performs color matching between the colors of an image which is input from the image input device 202 and is displayed on the image display device 203 with those of an image which is printed out by the image output device 204. An image processor 208 performs tone conversion processing, color conversion processing, and the like of an image to be output to the image output device 204. An image output unit 209 generates a signal required to output an image to the image output device 204.
Furthermore, the image processing apparatus 201 comprises a camera profile (or scanner profile) 210 for the image input device 202. Also, the image processing apparatus 201 comprises a monitor profile 211 for the image display device 203 and a printer profile 212 for the image output device 204. Note that the profiles 210 to 212 are stored as data files in a storage device such as a hard disk or the like.
The system shown in FIG. 2 has an advantage of easily coping with different devices by changing the profiles 210 to 212 to be used in correspondence with a change of input/output devices even when input and output devices connected are changed.
In order to allow the output device to reproduce colors that can be acquired by the input device, or in order to allow the input device to acquire colors which can be reproduced by the output device, the CMS uses a gamut technique that can absorb the influences of different gamuts between the input and output devices.
For example, Japanese Patent Application Laid-Open No. 6-225130 describes a general mapping method between input and output devices with different gamuts. That is, this reference describes a method of converting an input color space into a device-independent color space (uniform color space), and mapping colors, which cannot be reproduced by the output device of those of this color space in a minimum color difference direction, and a method of performing nonlinear mapping according to saturation in a constant lightness direction. A method described in Japanese Patent Application Laid-Open No. 4-40072 converts an input color space into a uniform color space or HVC color space as a device-independent color space, and checks if a color of this color space falls outside a gamut at the output destination. When the color falls outside the gamut, that color is mapped on a color which has the same lightness and hue values and a maximum saturation value.
However, the aforementioned mapping technique does not consider any shape of the gamut of the output device. For this reason, problems to be described below may be posed.
FIGS. 3A and 3B show an sRGB color space 300 as an input color space of an input device (e.g., a digital camera) and a gamut 301 or 302 of an output device (e.g., an ink-jet printer) using a CIELab color system.
The gamut 302 of the printer has poor saturation reproducibility in a low-lightness region compared to the gamut 301, and has a shape from which the gamut is cut away. In this way, the gamut of the printer often locally has a region with extremely poor color reproducibility, and may indicate a cutaway shape like region 1 shown in FIG. 3B or a bored shape like region 2.
An ink-jet printer performs color separation in consideration of graininess defined by printed dots, and a total ink droplet amount (to be referred to as “receptible ink amount” hereinafter) that can be received by a print medium per unit time and unit area. In order to reduce an ink supply amount to the print media, so-called undercolor removal (UCR) for substituting a gray part reproduced by respective color inks, i.e., cyan, magenta, and yellow, by black ink is made. In case of a print medium which has an extremely small receptible ink amount, the substitution amount of the black ink must be raised by increasing the UCR amount. However, when the UCR amount is increased, most of inks used in a low-lightness region are black ink, thus causing a saturation drop. Also, compared to gray reproduced by three or four colors, the printed dot density readily becomes coarse.
Furthermore, black ink uses a composition which allows ink to readily stay (dots do not spread) on the surface layer of a print medium using a pigment-based color material since it gives priority to reproduction of characters and line images on a plain paper sheet such as a copy print sheet or the like. On the other hand, color inks use a composition which allows ink to easily permeate a print medium so as to minimize a blur at the boundaries of different colors using a dye-based color material, since they give priority to color reproduction. In such system, since dots of black ink do not spread (do not grow) compared with color inks, the ink droplet amount ejected from a print head of the printer must be increased compared to color inks. For this reason, in consideration of the receptible ink amount of the print media in UCR, the substitution amount to black ink increases, thus further lowering the color reproduction performance of the low-lightness region.
For example, when a color 303 present on the boundary of the sRGB color space 300 shown in FIG. 3A is mapped toward a convergence point O of mapping so as to fall inside the gamut 301 of the printer, it is mapped as a color 304 on the boundary of the gamut 301. On the other hand, the color 303 is mapped on a color 305 in the gamut 302 of the printer shown in FIG. 3B. Compared to these colors, the color 305 is located in a considerably lower saturation region than the color 304 and is reproduced as a dull color. That is, when the same color 303 of the sRGB color space 300 is mapped, different colors are reproduced depending on the shape differences of the gamuts of the printers.
When black dye ink is used, region 1 with the cutaway shape corresponds to a mixed region of dye ink and pigment ink. Since pigment ink has poor permeability to a print medium, it is fixed near the surface of the print medium. For this reason, when a photo, e.g., hair or the like which has low saturation and a high density is printed, dark colors are color-reproduced as a region with a large pigment amount, relatively bright colors are color-reproduced as a region with a large dye amount. As a result, such photo seems as if it were suffering color unevenness.